1. Field of the invention
This invention relates to improvements in or relating to transducers or sensors, in particular position sensors for use in hostile environments. Such sensors are required to provide signals indicative of the position of a moving member with which they are associated, relative to some fixed datum, and to function reliably even in difficult environmental conditions. In particular, the invention relates to sensors functioning as level transducers, for example, for indicating the level of liquid within a boiler system.
2. Description of the prior art
Devices measuring physical position and its direct derivatives, velocity and acceleration, together with the many devices measuring parameters as indirect functions of a physical displacement, form what is probably the largest single category of transducer systems. Commercially available electrical position transducers include: limit and micro-switches, proximity detectors operating on either capacitive or inductive principles; potentiometers, linear and rotary variable differential transformers, optical encoders, both incremental and absolute, synchros, resolvers and Inductosyns (Trademark). Each device has its associated advantages and disadvantages and its place in the market.
Briefly, limit and micro-switches detect discrete events and are used typically as end-stops in machinery. Potentiometers provide a continuous position indication and, in spite of the wear problems associated with the sliding contact, are still widely used in equipment design because of their low cost. Proximity detectors have the advantage of being non-contacting devices and also detect discrete events, although they have been developed to give continuous position monitoring. Variable differential transformers are non-contacting devices and are widely used for moderate precision measurements. Optical encoders are available from the crudest miniature models through to devices measuring seconds of arc. Synchros and resolvers are used in moderate to high precision measurements. The Inductosyn (Trademark) gives resolver-like signals and is used principally for very high precision linear and rotary measurements.
The present invention is directed to screened inductance sensors. The basic principle of operation of such sensors allows them to be used in all but the crudest of the commercial applications described above, and also facilitates their development as high precision measurement systems. The output signal produced by these devices is a continuously variable amplitude modulated carrier frequency, which puts it in a general class with proximity detectors, variable differential transformers, resolvers and Inductosyns (Trademark).
Important areas of application for screened inductance sensors are those where continuous position measurements are required. As such, they are distinguished from inductive proximity detectors and variable differential transformers by the very high accuracy achievable, and their design flexibility. They are distinguished from resolvers and Inductosyns (Trademark) by simplicity, in particular their passive moving member. They may be distinguished from all other sensing devices by particularly favourable manufacturing costs.